1. Field of the Invention
The invention relates to a spray coating device, and in particular to a spray coating device for developing for improving coating uniformity.
2. Description of the Related Art
It is well known that photolithography is one of the most important steps in the semiconductor process. Any regions related to the structure of semiconductor devices, such as subsequently pattered and doped regions, are all defined by photolithography. Thus, the complexity of the semiconductor process depends on the times of photolithography performed and the number of masks used.
Although the photolithography is complicated, the principle thereof is quite simple. To perform the photolithography, a photo-sensitive material is first formed on a wafer. A parallel light coming from a light source passes through a glass-based mask and irradiates the photo-sensitive material. At the same time, a pattern on the mask is projected onto the photo-sensitive material. A selective photo-sensitive reaction is performed to accomplish a pattern transfer. In general, the photo-sensitive material is called a photoresist.
In addition to the light source, the mask and the photoresist layer, a developer is needed for developing during photolithography. Basically, the step of developing includes: (1) coating a photoresist layer; (2) exposing the photoresist layer; and (3) developing the exposed photoresist layer.
As to the developing, the exposed photoresist layer is developed to display a transferred pattern by cleaning out exposed parts of the photoresist layer with a neutral reaction. To avoid unexposed parts of the photoresist layer from being attacked by a developer, the conditions of developing, such as developing time, the concentration of the developer and developer temperature, must be strictly controlled. Typically, it is necessary to have enough time to expose a photoresist layer through a mask, such that exposed parts of the photoresist layer can completely neutrally react with the developer later. Moreover, developing time cannot be too long to surpass the line width tolerance of a desired photoresist pattern. On the other hand, the concentration of the developer can cause a direct effect during photolithography. The higher the concentration of the developer, the shorter the exposing time for the photoresist layer and the higher the throughput in the developing process. However, a poor resolution on a formed photoresist pattern occurs. Therefore, to obtain a desired photoresist pattern, the conditions of developing depend on the required line width and accuracy thereof. As to the temperature, the developer is maintained at a temperature of approximately 23.degree. C.
FIG. 1 is a schematic side view showing a conventional spray coating device 8 in an MKV development machine manufactured by TEL. Referring to FIG. 1, a developer 16 is transmitted to a spray head 14 via two inlet tubes 12, then sprayed on a wafer 18 from a plurality of spray holes 20. When spray coating the wafer 18, the wafer 18 is concurrently rotated one turn with a fixed axis 10 mounted at the center of the spray coating device 8 as a center.
Referring to FIG. 2, a top view of the spray coating device 8 is shown for better understanding how the distribution of the spray holes 20 is. As shown in FIG. 2, the spray holes 20 has a uniform distribution in number. During spray coating, each spray hole 20 emits the same amount of developer 16. However, since the center part of the rotated wafer 18 has faster moving speed than the surrounding part thereof, the center part is applied with more amount of developer than the surrounding part. That is, the center part and the surrounding part of the rotated wafer 18 have different amounts of developer 16 sprayed thereon.
To further describe different amounts of developer 16 sprayed on the center part and the surrounding part of the wafer 18, the distributions of path length and spray amount of developer on a rotated wafer, respectively, according to the prior art are shown in FIGS. 3A-3B. Referring to FIG. 3A, x-axis represents a distance away from the center of the rotated wafer with its origin severing as the center while y-axis represents a corresponding path length of developer sprayed on the rotated wafer. As shown in FIG. 3A, the path length is equal to 2.pi.r, wherein r is a distance away from the center of the rotated wafer. Obviously, at the center of the rotated wafer, the corresponding path length is equal to 0. Referring to FIG. 3B, x-axis represents a distance away from the center of the rotated wafer 18 with its origin severing as the center while y-axis represent a corresponding spray amount per unit length on the rotated wafer. If each spray hole has a spray amount of .DELTA.m, the spray amount per length on a corresponding path length is equal to .DELTA.m/2.pi.r. It is reasonable that the spray amount at the center of the rotated wafer is .DELTA.m because the center is static. As can be seen from FIG. 3B, a path length closer to the center of the rotated wafer has a larger spray amount per unit length. Therefore, the center part of the rotated wafer has a larger spray amount per unit length than the surrounding part thereof. As a result, the center part of the rotated wafer is more easily over developed while the surrounding part thereof is more easily insufficiently developed, resulting in low manufacturing yield.
Additionally, when some of spray holes, especially corresponding to the surrounding part of the rotated wafer are congested, the tolerance thereof becomes smaller, obvious leading to a poor uniformity of spray coating.